Known diagnostic methods provide a check of individual sensors or actuators or their operating chains. U.S. Pat. No. 4,794,790 describes a diagnostic method for a tank-venting valve wherein an idle actuator operates to close at the same time as the tank-venting valve opens. The effects of the drives of both actuators should cancel each other in a trouble-free state of operation.
It is known to measure the temperature increase in the intake manifold when there is an active exhaust-gas recirculation for diagnosing the latter.
Furthermore, it is known from the xe2x80x9cAutomotive Handbookxe2x80x9d, Fourth Edition (1996) published by the Society of Automotive Engineers to provide an integrated diagnosis for engine control apparatus wherein, inter alia, a substitute signal is formed from engine rpm and throttle flap angle when there is a fault of the load sensor (air mass, air quantity, intake manifold pressure).
It is an object of the invention to provide an improvement of the self diagnosis of engine control systems and especially engine control systems for internal combustion engines having variable exhaust gas recirculation.
According to the invention, a determination of the cause for a fault in the formation of the air/fuel mixture for an internal combustion engine includes the following steps: detecting different operating parameters of the engine; forming at least three load signals, which represent the respective air quantities flowing into the engine, on the basis of at least partially different operating parameters; forming different pairs of two load signals each; determining the deviations of each of the two signals of one pair with respect to each other for different pairs; attributing various causes to different combinations of pairs wherein deviations have been determined within the pairs.
In one embodiment, at least three of the following quantities are detected as operating parameters:
(a) signal of an air-mass sensor;
(b) signal of an rpm transducer;
(c) signal of an intake manifold pressure sensor;
(d) signal of an exhaust-gas probe;
(e) signal of a fuel pressure sensor.
In connection with the above, the following signals can be formed:
(a) a first load signal rl_hfm on the basis of the signal of the air-mass sensor;
(b) a second load signal rl_ps on the basis of an intake manifold pressure transducer and an rpm transducer; and,
(c) a third load signal rl_rk on the basis of the following: the injection time, the signal of a fuel pressure sensor; and, an exhaust-gas probe signal.
Furthermore, the following pairs of two load signals each are formed:
(a) a first pair from first and second load signals;
(b) a second pair from first and third load signals; and,
(c) a third pair from second and third load signals.
A defective tank-venting valve is deemed to be the fault cause when there are deviations in each pair of a combination of first, second and third pairs.
A defective intake manifold sensor or a defective exhaust-gas recirculation valve or a clogged air filter are deemed to be the fault cause when there are deviations in the first and in the third pairs of a combination of first, second and third pairs.
Deviations in the first and second pairs of a combination of first, second and third pairs are attributed to a defective air-mass sensor or a leak in the exhaust-gas module.
Deviations in the second and in the third pairs of a combination of first, second and third pairs are attributed to a defective fuel pressure sensor.
To distinguish a defective intake manifold pressure sensor from a defective exhaust-gas recirculation valve or a clogged air filter, supplementary, a first value can be stored for the pressure forward of the throttle flap from the following: the intake manifold pressure (rearward of the throttle flap), the throttle flap angle and the rpm during the operation of the engine; and,
a second value for the intake manifold pressure at standstill (corresponding to the pressure forward of the throttle flap) of the engine can be detected and can be compared to the first value.
When both intake manifold pressure values are the same, the intake manifold pressure sensor can be indicated as being defective and when the first value is exceeded by the second value, the air filter can be indicated as being clogged; and, when the second value drops below the first value, the exhaust-gas recirculation valve can be indicated as being defective.
The advantages of the invention are that the invention permits faults to be distinguished of the following: the air-mass sensor; the intake manifold pressure sensor; the exhaust-gas recirculation valve; the tank venting; the fuel pressure sensor as well as the detection of leakage air in the intake manifold or of a clogged air filter.
A special advantage of the invention is that the cause of a defective mixture formation can be determined with relatively simple functions.